Create paint pour with fluids, needs to fill with a round mesh
Set the viscosity high (either set the Base and Exponent manually in Fluid World properties or by using the Fluid World 'Viscosity Preset' for Honey (which should be somewhere close to paint)) and then use a Fluid Control object to keep the pool of liquid in the desired shape. 

See How to melt an object then reverse the melt into another object for an example of using Control objects to direct the fluid into a specific shape, using a thin cylinder to shape the pool.